Fleetwood Mac began life as merchants of pure blues, deeply indebted to the likes of Elmore James and Robert Johnson. Their first two albums – Fleetwood Mac and Mr. Wonderful, both from 1968 – adhere to straightforward blues, straying very little from formula. But by the time of their third album, Then Play On, things were beginning to steer toward more interesting directions.

By 1969, Fleetwood Mac were taking their basic blues template and adding a lot more to the mix. The addition of guitarist Danny Kirwan gave another dimension to their sound, but it was the writing styles of both Green and Kirwan that would lift things out of the tried and true.

Then Play On kicks off with the haunting blues stride "Coming Your Way." Written by Kirwan, the song uses the blues as a jumping-off point, but weaves in an ethereal quality – a combination of Green's lead guitar and Fleetwood's rumbling percussion that makes for a perfect album opener. Green's haunting "Closing My Eyes" sneaks in next, with nothing but vocals and guitars. It's a dark song that evokes the soul of the blues without relying on the form.

Kirwan's "When You Say" goes the acoustic route, while Green's "Show Biz Blues" turns into a country-blues raver featuring some great acoustic slide guitar. And on the instrumental "Underway," Green gets to cut loose with some beautiful lead work that drifts and lifts itself higher and higher as the songs moves on.

Side two begins with one of the band's most beautiful Mac songs, Kirwan's "Although the Sun Is Shining," a short but sweet acoustic number. The next track, "Rattlesnake Shake," takes an opposite approach. It's blues, but far removed from the traditional route. Just as Cream twisted the music into other shapes, Fleetwood Mac created something stunningly new from old forms. The song was a centerpiece of the group's live shows at the time, sometimes carrying on for nearly 30 minutes.

"My Dream" is an instrumental showcase for Kirwan, who also penned "Like Crying," an Everly Brothers-style song that sets up a duet between himself and Green. "Before the Beginning" ends the original LP, sort of like how it started, with haunting guitars and vocals, ushering listeners out the same way they were invited in. It's another fine Green composition that hints at the inner turmoil that would eventually be his undoing.

Rolling Stone didn't think too much of Then Play On at the time, calling it "slow and wandering," concluding that "on this album, they fall flat on their faces." True, the record has slow spots – too many mid-tempo songs and instrumentals take away from the flow. But in retrospect, the LP shows the band in search of a new identity, and remains one of its most intriguing artifacts.

Listen to Fleetwood Mac Perform 'Before the Beginning'

But that's not the entire story of this chapter in the band's long career. Two significant singles were released after the album came out: "Oh Well" and "The Green Manalishi (With the Two Prong Crown)." The former was issued shortly after the album and was in fact added to later U.S. pressings of Then Play On. It remains one of the band's greatest numbers. The monster riff alone is a head-turner, but the barn-burning fury the band delivers is mind boggling.

And Green's autobiographical lyrics take things to another level: "Now, when I talked to God I knew he'd understand / He said, 'Stick by me and I'll be your guiding hand / But don't ask me what I think of you / I might not give the answer that you want me to.'" The single climbed to No. 2 in the U.K. and has remained a part of Fleetwood Mac's live set ever since.

But "The Green Manalishi (With the Two Prong Crown)" is where it all ends – Green's final recording with the band. This is where his vision of the blues and the world in general comes into evil and haunting focus. His heavy LSD use comes through in both sound and words. By the final coda, Green has basically lost his mind – primal scream therapy set to a menacing blues dirge, if you will. Not since Del Shannon, another troubled soul, lost his mind at the end of "Keep Searchin" in 1964 has anyone sounded this unhinged. (The song was given a second life when Judas Priest recorded a more straightforward version a decade later.)

Then Play On helped to establish Fleetwood Mac in the U.S., where it gained a foothold in the underground scene. Green would soon leave the band under mysterious circumstances involving religion and drugs. The Mac would rebound with a series of albums in the early '70s before becoming one of the biggest bands in the world when two Americans joined for another chapter in their awesome career.